1. Field of the Invention
The invention relates to compositions based on silane-crosslinking prepolymers, to their methods of manufacture and to their use as adhesive and sealant materials, in particular for adhesive bonding of substrates.
2. Description of the Related Art
Polymer systems having reactive alkoxysilyl groups are well known. On contact with water and/or atmospheric humidity, these alkoxysilane-terminated polymers are able to condense with each other even at room temperature by elimination of the alkoxy groups. One of the most important applications for such materials is the manufacture of adhesives, particularly adhesive systems that are elastic.
In their cured state, adhesives based on alkoxysilane-crosslinking polymers not only exhibit good adherence properties to some substrates, but also very good mechanical properties, since they combine high tensile strength at break with high elasticity. A further decisive advantage of silane-crosslinking systems over numerous other adhesive and sealant technologies (over isocyanate-crosslinking systems, for example) is the fact that the prepolymers are toxicologically benign.
There are many applications where there is preference for single-component systems (1K systems) that cure on contact with atmospheric humidity. The decisive of single-component systems is particularly the fact that they are very easy to use, since there is no requirement here for the user to mix various components of the adhesive. In addition to saving work/time and safely obviating any dosage errors, single-component systems also do not impose a need to process the adhesive/sealant material within a usually fairly tight time window, as is the case with multi-component systems, after the multiple components have been mixed together.
What is disadvantageous with these systems as represented in the prior art, is particularly the low reactivity of the corresponding MS or SPUR polymers toward moisture, because this necessitates aggressive catalysis. The corresponding mixtures therefore typically contain appreciable amounts of toxicologically suspect tin catalysts.
The use of so-called α-silane-terminated prepolymers is advantageous here because they have reactive alkoxysilyl groups which are attached with a methylene spacer to an adjacent urethane unit. Compounds of this class are highly reactive in that they require neither tin catalysts nor strong acids or bases to provide high rates of cure on contact with air. Commercially available α-silane-terminated prepolymers are GENIOSIL® STP-E10 or -E30 from Wacker-Chemie AG.
However, most commonly used silane-crosslinking systems are disadvantageous, because although they do achieve moderate tensile strengths, typically on the order of 1-4 MPa, they only achieve tensile strengths >5 MPa with very great difficulty, if at all. Yet such higher tensile strengths are needed for numerous applications, for example for numerous structural adhesives, high-performance adhesives for DIY or home improver applications, or else for the adherent bonding of windshields. In addition to a high level of tensile strength, however, the adhesives in question should also exhibit high elasticities, i.e., elongations at break >100%.
One possible way to enhance the tensile strength of silane-crosslinking systems is to incorporate short-chain diols in silane-crosslinking polyurethanes, as described in WO 05/000931. However, even this measure only gives systems having moderately improved tensile strengths.
One of the few possible ways to achieve tensile strengths >5 MPa is to use carbon black as filler, as described in WO 02/090411 for example. However, the use of carbon black has the massive drawback that only black adhesives are obtainable. This is only acceptable to the particular users in very few adhesive applications. In addition, even the carbon black-containing formulations described in WO 02/090411 fail to achieve tensile strengths >6.5 MPa.
A further possible way to achieve very high tensile strengths with silane-crosslinking systems is described in WO 2011/026658. High tensile strengths are achieved here by the use of silane-terminated polyurethanes based on extremely short-chain polyols having molar masses <1000 g/mol. One consequence of this is that the resulting silane-crosslinking polymers possess not only a very high density of urethane and/or urea units capable of hydrogen bonding but also a correspondingly high number of crosslinkable silane end groups. However, systems of this type inevitably have three system-inherent disadvantages. First, the preparation of prepolymers having a high concentration of silane-crosslinking groups requires correspondingly large amounts of silane. These are generally the most cost-intensive prepolymer constituents, which causes a corresponding rise in the raw-material costs for these products. Second, the likewise necessary high concentration of urethane and/or urea groups which is needed to achieve high tensile strengths leads to very high prepolymer viscosities. So it is correspondingly problematic to compound these prepolymers into ready-formulated adhesives and also to work with these usually likewise comparatively highly viscous end products. Finally, the short chain lengths and high crosslink densities of the corresponding polymers lead to hard but also very inelastic materials having elongations at break far below 50%.
In addition to these disadvantages as regards properties, the polymer types described in WO 2011/026658 further also have the disadvantage of being commercially available to only a very limited extent, if at all.
A third possible way to achieve very high tensile strengths with silane-crosslinking systems is described in EP 1 957 553 A. The adhesive formulations described therein contain specific silane-terminated polyurethanes having an average chain length of about 7000 g/mol, pyrogenic silica of low BET surface area, and monomeric silanes, wherein the pyrogenic silica is used in amounts between 7 and 20 wt % and the monomeric silanes are used in amounts >10 wt %.
However, the comparatively low elongation at break for the corresponding adhesives is also a disadvantage. In addition, the systems described in EP 1 957 553 A have the disadvantage of being solely reliant on silane-crosslinking types of polymers which are not commercially available.